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How I did it on my mid-2012 27" iMac with OS X El Capitan

Thanks to the original post and all comments.

Finally I was able to upgrade the SSD firmware on my mid-2012 27" iMac. The iMac has its original HD and a Crucial m4 256G SSD installed, and is running OS X El Capitan.

The main problem I was having is that the firmware of the iMac itself had been upgraded, and it refused to boot from an external USB drive. Previous comments mentioned using the original HD instead, so I thought I could give that a try.

Another problem is that I couldn't download the refit-syslinux image file -- it's not accessible any more.

The basic idea of my solution is to use rEFInd as the boot loader for syslinux installed on the internal HD. I followed these steps:

  1. Download and install rEFInd. Note that for El Capitan, you need to use the Terminal in Recovery mode, because of SIP. The rEFInd installation guide has details on this.
  2. Download Crucial's manual boot image containing the firmware, as an ISO file.
  3. Use UNetBootIn to create a bootable USB drive using Crucial's ISO. This USB drive will contain both syslinux and Crucial's image (boot2880.img). The only problem is my iMac refused to boot from it ...
  4. Use Disk Utility to erase the internal HD (not the SSD, the original HD). I used Master Boot Record and FAT32 file system.
  5. Boot iMac into Recovery mode. Open terminal. Use dd to copy the entire volume of the USB drive to the internal HD. For me the command was dd if=/dev/rdisk2s1 of=/dev/rdisk1s1 bs=4m. It's a slow process because it copies the whole volume byte-by-byte (I used a 16G USB drive so that's 16G copied), even the files only takes less than 100M. So, use a small USB drive if you have one.
  6. Remove USB drive and reboot iMac. rEFInd should be able to find a Linux OS on the internal HD. Boot from the Linux OS. Choose the second "default" from the menu (not sure if the first one will work but you can try). It should boot into the firmware upgrader and you can follow the steps from there.
  7. Hope this helps!


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